Rate adaptive XDSL communication system and method

ABSTRACT

A rate adaptive digital subscriber line (XDSL) communication system and method are disclosed. An XDSL link for XDSL equipment is trained at a data rate set by a baud rate and a constellation size. Actual operating characteristics of the XDSL equipment are then obtained from parameters for the trained XDSL link. One or more rate adaptive data tables storing empirical data for performance of the XDSL equipment are accessed. Then, it is determined whether the trained data rate will provide a desired bit error rate using the actual operating characteristics and the empirical data from the accessed one or more of the rate adaptive data tables. In one embodiment, if the trained data rate will not provide the desired bit error rate, a new data rate is selected using the empirical data and the actual operating characteristics. Then, the XDSL link is trained using the new data rate, and the process is repeated. Further, in one embodiment, the actual operating characteristics used comprise the actual receiver gain and signal-to-noise ratio, and the empirical data comprises specified receiver gains, signal-to-noise ratios, baud rates and constellation sizes used to achieve a desired bit error rate.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Application Ser. No.09/219,148, filed Dec. 22, 1998, U.S. Pat. No. 5,999,540, By David W.McGhee and entitled “Rate Adaptive XSDL Communication System andMethod.”

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to XDSL communication systemsand, more particularly, to a rate adaptive XDSL communication system andmethod.

BACKGROUND OF THE INVENTION

In XDSL communication systems, there can be provisions made for adaptingthe rate at which data is communicated across the XDSL link. RateAdaptive DSL (RADSL) is one example of such a system. In general, thedata rate across an XDSL link is determined by the baud rate and thenumber of bits per symbol, or constellation. Thus, the data rate can beadjusted by adjusting either the baud rate or the size of theconstellation.

Typically, conventional XDSL chip sets implement rate adaptivealgorithms that provide a choice of rate based solely upon thesignal-to-noise ratio (SNR). However, these schemes typically take toolong to functions. Further, they often do not work because a givensignal-to-noise ratio does not guarantee the desired performance (e.g.,10⁻⁷ bit error rate) at the desired rate. This is true in part becausethe chip set is assuming performance based upon relatively constantnoise, when, in fact, real world physical connections do not typicallyexperience constant patterns of noise. Thus, the conventional rateadaptive algorithms are incorrect on some types of noise models and donot accurately select the appropriate rate.

SUMMARY OF THE INVENTION

In accordance with the present invention, a rate adaptive XDSLcommunication system and method are provided that provide significantadvantages over conventional XDSL communication systems.

According to one aspect of the present invention, a rate adaptivedigital subscriber line (XDSL) communication system and method train anXDSL link for XDSL equipment at a data rate set by a baud rate and aconstellation size. Actual operating characteristics of the XDSLequipment are then obtained from parameters for the trained XDSL link.One or more rate adaptive data tables storing empirical data forperformance of the XDSL equipment are accessed. Then, it is determinedwhether the trained data rate will provide a desired bit error rateusing the actual operating characteristics and the empirical data fromthe accessed one or more of the rate adaptive data tables. In oneembodiment, if the trained data rate will not provide the desired biterror rate, a new data rate is selected using the empirical data and theactual operating characteristics. Then, the XDSL link is trained usingthe new data rate, and the process is repeated. Further, in oneembodiment, the actual operating characteristics used comprise theactual receiver gain and signal-to-noise ratio, and the empirical datacomprises specified receiver gains, signal-to-noise ratios, baud ratesand constellation sizes used to achieve a desired bit error rate.

It is a technical advantage of the present invention to use receivergain and signal-to-noise ratio (SNR) together to choose a data rate.This selection makes the communication immune to characteristics of thenoise because, for a given signal to noise ratio and receiver gain, theperformance of the XDSL transceiver can be empirically determined andstored in rate adaptive data tables.

It is another technical advantage of the present invention that the rateadaptive data tables can be derived from a whole end-to-end system withsimulated noise. This means that the performance of the system is knownfor a given signal-to-noise ratio and receiver gain and that timeconsuming bit error rate tests based upon a known bit test pattern donot have to be used.

It is a further technical advantage of the present invention to allowthe communication system to verify the current data transfer rate and touse the rate adaptive data tables to estimate the actual signal-to-noiseratio margin and to modify the amount of margin available.

Additional technical advantage of the present invention should beapparent from the drawings and specification.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention and advantagesthereof may be acquired by referring to the following description takenin conjunction with the accompanying drawings, in which like referencenumbers indicate like features, and wherein:

FIG. 1 is a block diagram of one embodiment of a rate adaptive XDSLcommunication system;

FIG. 2 is a diagram of one embodiment of a rate adaptive data table;

FIG. 3 is a flow chart of one embodiment of a method for generating arate adaptive data table; and

FIG. 4 is a flow chart of one embodiment of a method for selecting anadaptive data rate.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram of one embodiment of a rate adaptive XDSLcommunication system, indicated generally at 10. As shown, system 10comprises an XDSL termination unit (XTU) which, for example, can be anADSL modem. XDSL termination unit 12 communicates across an XDSL link 14with network equipment such as that located at a telephone systemcentral office or other local loop termination point. In general, XDSLtermination unit 12 comprises a processor 14 and a physical layer chipset 16. Physical layer chip set 16 accomplishes communication of bitsacross the XDSL link 14 under the control of processor 15. Further, asshown, XDSL termination unit 12 comprises a memory 18. Memory 18 cancomprise on-board random access memory (RAM), FLASH memory or otherappropriate local storage. According to the present invention, memory 18stores a plurality of rate adaptive data tables 20 which contain look-upinformation accessible by a processor 15 for setting and analyzing thedata rate across XDSL link 14.

FIG. 2 is a diagram of one embodiment of a rate adaptive data (RAD)table 20. Rate adaptive data table 20 maps data rate information for anend-to-end XDSL link against measured variables. Thus, given certainvalues for the variables, the performance of the link is known. In theembodiment of FIG. 2, rate adaptive data table 20 maps the constellationsize needed to maintain a desired bit error rate (e.g., 10⁻⁷ BER)against receiver gain (RX GAIN) and signal-to-noise ratio (SNR) for agiven baud rate. Thus, as shown, rate adaptive data table 20 results ina map showing different constellation sizes (e.g., 16, 64, 256, 256U)that can be used at the given baud rate to achieve the desired bit errorrate for a given combination of receiver gain and signal-to-noise ratio.In FIG. 2, the thresholds drawn across rate adaptive data table 20indicate where the constellation size change to maintain the bit errorrate at the desired level (e.g., 10⁻⁷).

There are, of course, other ways to generate rate adaptive data tablesto record the performance of an XDSL link. For example, the table couldattempt to describe the receiver gain and signal-to-noise ratio pointsthat follow the desired bit error rate level. This alternative might bemore precise than that of FIG. 2, but might also be more difficult tocreate and manipulate through a software process.

Returning to the embodiment of FIG. 2, rate adaptive data table 20 canbe used, given a particular receiver gain, signal-to-noise ratio andbaud rate, to make an appropriate selection as to the constellation sizeto be used such that the desired bit error rate (e.g., 10⁻⁷) isachieved. Further, as shown, a given point within the table can beselected such that there is a given amount of margin (e.g., 3 dB)between the selected setting and the next constellation threshold.

FIG. 3 is a flow chart of one embodiment of a method for generating arate adaptive data table such as that shown in FIG. 2. For appropriateresults, this empirical test and data generation should involveequipment identical to that which will eventually be installed in thefield and should involve the physical layer chip set that will be used.In this case, based upon the empirical testing, the operation of theequipment once installed in the field can be accurately predicted.

As shown in FIG. 3, in step 30, a baud rate is selected for an empiricaltest of an end-to-end XDSL link. Then, in step 32, the physical layer istested at various data rates against different line lengths and noisetypes. Using these test conditions, in step 34, empirical data forreceiver gain, signal-to-noise ratio and constellation size can begenerated for a given baud rate. Then, in step 36, a rate adaptive datatable can be built for the selected baud rate. Thus, by using theprocess of FIG. 3, a series of rate adaptive data tables, such as thatof FIG. 2, can be generated for XDSL equipment. Then, by access to suchtables and using a known receiver gain and signal-to-noise ratio, theXDSL equipment can select an optimal baud rate and constellation sizefor achieving desired performance (e.g., 10⁻⁷ bit error rate).

FIG. 4 is a flow chart of one embodiment of a method for selecting anadaptive data rate. As shown, in step 40, the XDSL link is trained atthe provisioned rate. Then, in step 42, it is determined whether thetrain was successful. If not, then the XDSL link is retrained at a lowerrate in step 44. In some cases, the lowest possible rate may be chosento ensure that a successful train is obtained. In other cases, the nextlower data rate may be chosen. In any event, after a successful train,the receiver gain and signal-to-noise ratio are retrieved from thetrained parameters in step 46. Then, in step 48, a look-up is performedin the appropriate rate adaptive data table. For example, the rateadaptive data table could be a table for the trained baud rate like thatshown in FIG. 2.

This look-up uses known parameters of the trained link (e.g., receivergain and signal-to-noise ratio) to determine whether the link isoperating at an optimal data rate and, if not, what change should bemade. In this context, “optimal” is used to refer to whether the currentdata rate best achieves the desired bit error rate and margin. Thus, instep 50, it can be determined whether the link is operating at anoptimal data rate (i.e., the optimal baud rate and constellation size).If the data rate is optimal, then, in step 52, the XDSL equipmentcontinues to operate at the current data rate.

Otherwise, it can be determined what change should be made. In step 54,it is determined whether there should be a move to a higher baud rate.If so, the link is trained at the higher baud rate and returns to step42. If not a higher baud rate then, in step 58, it is determined whethera different constellation size should be used at the same baud rate. Ifso, the link is trained in step 60 at the different constellation size,and the process returns to step 42. If there is neither a move to ahigher baud rate or a different constellation size, then in step 62, thelink is trained at a lower baud rate. After step 62, the processcontinues at step 42 to determine whether the train was successful.

In general, according to the present invention, an appropriate data ratecan selected using known current loop conditions. In particular, usingthe signal-to-noise ratio and receiver gain, an empirically generatedrate adaptive table data allows the XDSL equipment to select a data ratethat achieves desired performance regardless of noise or loop conditions(e.g., loop link, bridge taps). Another important part of the process isthe handling of the data rate transitions. When the baud rate needs tobe changed going down, the equipment can start at one of the higherconstellation sizes for that baud rate. When baud rate needs to go up,the equipment can start at the lower constellation size for the nexthigher baud rate. It should be noted that it is possible, usingperformance testing, to tune the cross baud guessing to eliminate ratesthat will probably not be used or are actually slower than the higherrate at a lower baud rate.

Although the present invention has been described in detail, it shouldbe understood that various changes, substitutions and alterations can bemade thereto without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A method for rate adaptive XDSL communication,comprising: training an XDSL link for XDSL equipment at a first datarate; determining whether the first data rate will provide a desired biterror rate based on parameters of the XDSL link and performance data forthe XDSL equipment, wherein the performance data are determined beforetraining at the first data rate and the parameters are determined aftertraining at the first data rate; selecting a second data rate if thefirst data rate will not provide the desired bit error rate; andtraining the XDSL link at the second data rate.
 2. The method of claim1, wherein selecting the second data rate comprises accessing theperformance data based on the parameters to determine a baud rate and aconstellation size for achieving the desired bit error rate.
 3. Themethod of claim 1, wherein the XDSL equipment comprises an XDSL modem.4. A method for determining performance characteristics for XDSLequipment, comprising: training an XDSL link for XDSL equipment at aplurality of data rates; determining performance data for the XDSL linkfor each of the data rates; and storing the performance data for each ofthe data rates.
 5. The method of claim 4, further comprising accessingthe performance data to determine a data rate based on a receiver gain,a signal-to-noise ratio, and a desired bit error rate.
 6. The method ofclaim 4, wherein the performance data indicate a baud rate and aconstellation size required to achieve a desired bit error rate given areceiver gain and a signal-to-noise ratio.
 7. The method of claim 4,wherein training further comprises training the XDSL link using aplurality of line lengths coupled to the XDSL equipment.
 8. The methodof claim 4, further comprising subjecting the XDSL link to a pluralityof noise types.
 9. The method of claim 4, wherein the XDSL equipmentcomprises an XDSL modem.
 10. The method of claim 4, wherein storing theperformance data comprises storing the performance data in productionXDSL equipment, wherein the production XDSL equipment is similar to theXDSL equipment.
 11. A device for maintaining rate adaptive performancedata for XDSL equipment, comprising: a memory storing performance datadescribing bit error rates for associated data rates based on operatingparameters; and an interface operable to: receive actual operatingparameters and a desired bit error rate; access the performance datausing the actual operating parameters and the desired bit error rate todetermine a data rate for achieving the desired bit error rate; andoutput the data rate.
 12. The device of claim 11, wherein the operatingparameters comprise a receiver gain and a signal-to-noise ratio.
 13. Thedevice of claim 11, wherein the performance data indicate a baud rateand a constellation size required to achieve a desired bit error rategiven a receiver gain and a signal-to-noise ratio.
 14. The device ofclaim 11, wherein the XDSL equipment comprises an XDSL modem.
 15. Anapparatus for rate adaptive XDSL communication, comprising: an XDSLlink; a memory storing performance data for the apparatus; and aprocessor operable to: train the XDSL link at a first data rate;determine whether the first data rate will provide a desired bit errorrate based on parameters of the trained XDSL link and the performancedata; select a second data rate if the first data rate will not providethe desired bit error rate; and train the XDSL link at the second datarate.
 16. The apparatus of claim 15, wherein the performance data aredetermined before training at the first data rate and the parameters aredetermined after training at the first data rate.
 17. The apparatus ofclaim 15, wherein the processor is further operable to select the seconddata rate by accessing the performance data based on the parameters todetermine a baud rate and a constellation size for achieving the desiredbit error rate.
 18. The apparatus of claim 15, wherein the apparatuscomprises an XDSL modem.
 19. An apparatus for rate adaptive XDSLcommunication, comprising: an XDSL interface operable to couple to anXDSL link; a memory storing performance data for the apparatus; and aprocessor operable to: train the XDSL link at a first data rate;determine whether the first data rate will provide a desired bit errorrate based on parameters of the trained XDSL link and the performancedata; select a second data rate if the first data rate will not providethe desired bit error rate; and train the XDSL link at the second datarate.
 20. The apparatus of claim 19, wherein the performance data aredetermined before training at the first data rate and the parameters aredetermined after training at the first data rate.
 21. The apparatus ofclaim 19, wherein the processor is further operable to select the seconddata rate by accessing the performance data based on the parameters todetermine a baud rate and a constellation size for achieving the desiredbit error rate.
 22. The apparatus of claim 19, wherein the apparatuscomprises an XDSL modem.